The present disclosure is related to imaging systems, and more specifically to multiple-beam laser sources and imaging systems used in printers, copiers, facsimile machines and the like employing same.
There are several types of modern imaging (e.g., printing and copying) devices, typically separated by the type of system (or imaging engine) used to produce an image. One example is an electrophotographic marking system, which forms the imaging engine for many modern printers, copiers, facsimile machines, and other similar devices.
In a typical electrophotographic marking system, a light source such as a laser is caused to expose a photosensitive surface, such as a charged photoreceptor, with a representation of a desired image. The photoreceptor is discharged where exposed, creating an electrostatic latent image on the photoreceptor's surface. Toner particles are then selectively applied to the photosensitive surface where exposed (or alternatively where not exposed), forming a latent (toner) image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner is then fused to the substrate, usually by using heat and/or pressure, to thereby create a permanent printed image on the substrate. The surface of the photoreceptor is then cleaned of residual toner and recharged in preparation for subsequent image production.
The foregoing broadly describes a monochrome (black and white) electrophotographic marking system. Electrophotographic marking can also produce polychromatic (color) images in several different ways, for example by repeating the above process once for each color of toner that is used to make a composite color image. In one example of a color process, called a READ IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say cyan. The resulting electrostatic latent image is then developed with cyan toner to produce a cyan toner image. The charge, expose, and develop process is repeated, using the same photoreceptor, for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say black. The various latent images and color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate. Alternatively, a multiple exposure station system can be employed, requiring a separate charging, exposing, and developing station for each color of toner.
One way of exposing a photoreceptor in systems such as those described above is to use a laser array source subsystem and a Raster Output Scanner (ROS) subsystem. A laser array source subsystem is typically comprised of a multiple source laser array and associated optics for collimating, focusing, etc. the laser beams output by the array. A ROS subsystem is typically comprised of a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of operation, a collimated laser beam is reflected from the facets of the polygon and passed through imaging elements that project the laser beam into a finely focused spot of light on the photoreceptor's surface. As the polygon rotates, the source beam traces a path on the photoreceptor surface referred to as a scan line. By synchronizing motion of the photoreceptor with the polygon rotation, the spot raster scans (i.e., one line after another) the surface of the photoreceptor. By modulating the laser beam with image information a desired latent image is produced on the photoreceptor. The direction of the sweeping beam is referred to as the scan direction, while the generally perpendicular direction of motion of the photoreceptor is called the process direction.
One measure of the quality of a printing system is its scan resolution. Scan resolution is essentially a measure of how finely an individual pattern of printed pixels may be positioned by a printing system. Modern printing systems reach scan resolutions as high as 2400 dots-per-inch (dpi). This produces very smooth curves, solid blocks of color, smooth color transitions, and so forth. To practically achieve this resolution, the light source used with the ROS system is an integrated array capable of simultaneously producing multiple, individually addressable, spaced-apart light beams. Current state-of-art printing systems use an integrated array of as many as 32 laser light sources for ROS printing. A vertical cavity surface emitting laser (VCSEL) two-dimensional array is a typical integrated source used in ROS-type electrophotographic marking systems.
An exemplary 8-row-by-4-column integrated array provides columns of sources at the laser array with each source spaced apart in the scan direction by 30-40 microns, the sources in each column spaced apart by 20-30 microns, and each column shifted downward in the process direction from the previous column by 5-10 microns. Such an integrated array employed in a typical ROS system produces a spot pattern on the photoreceptor such that each spot is spaced apart in the scan direction by 450-550 microns, spot-to-spot spacing in the process direction of 42.333 microns, and each column of spots shifted downward in the process direction from the previous column by 10.583 microns. FIG. 1 is a beam layout 10 viewed in plan view of the exit facet of an integrated multi-beam source according to a known array for simultaneously producing 32 individually addressable, spaced-apart light beams 12. FIG. 2 is a spot pattern 14 produced by the array of FIG. 1 (post-optics) at the photosensitive surface (image plane) according to the prior art in which each of the 32 lasers produce a spot 16.
FIG. 3 is an exemplary prior art imaging apparatus 20, such as a printer, copier, or the like. While an in-depth review of a complete apparatus 20 is beyond the scope of the present disclosure, a detailed description of such a device may be found, for example, in U.S. Pat. No. 7,466,331 and U.S. Pat. No. 7,236,280, each of which being incorporated herein by reference.
Briefly, a typical apparatus 20 comprises a raster output scanner (ROS) sub-system 22, an array source subsystem 24, rotating polygon mirror and lens assembly 26, and controller 28 which manages these elements to produce a light beam(s) “b” which is made incident on the photosensitive surface of a rotating photoreceptor 30. Toner is selectively picked up by the photoreceptor 30 where exposed by beam b to form a latent image, which is then transferred and fused to a paper substrate 32. The photoreceptor is cleaned and recharged, and the process repeated.
With reference to FIG. 4, raster output beam array scanning sub-system 22 of a type known in the art is shown. As mentioned, scanning sub-system 22 typically includes a polygon mirror and lens assembly subsystem 26, which itself comprises a rotating polygon mirror 34, and numerous optical elements 36 which, among other functions, serve to provide a compact optical path and optical beam conditioning and correction for the beam(s) produced by integrated multi-beam laser source 25 of array source subsystem 24. Array source subsystem 24 generates one or more light beams which together form a beam array 38. Optical elements focus and collimate the beam array 38, and an aperture 40 defines the width of the collimated beams from the array.
A beam splitter 42 may be disposed in the optical path of beam array 38. Beam splitter 42 allows some amount of the light energy of beam array 38 to pass therethrough to proceed to polygon mirror 34, optical elements 36 and ultimately to photoreceptor 30. Beam splitter 42 redirects the balance of the light energy to a beam monitor 44 such as a photodiode optical power monitor. The basis for splitting the beam array in this fashion is to provide a view of the beam array which can be used to adjust power, time pulse sequencing, beam position, and other attributes of array source subsystem 24 and the process of generating beam array 38. This monitoring is particular important in high-resolution, multi-beam system in order to obtain optimum output quality.
However, there is an ever-present demand for improved imaging quality. Electrophotographic marking systems are comprised of a number of optical elements. Unavoidable imprecision in the shape and/or mounting of these optical elements, wear, environmental changes, etc. inevitably introduce anomalies in the quality of the scan line on the photoreceptor, leading to reduced quality imaging. One such anomaly is slight variation in scan line spacing on the photoreceptor. Such spacing variation, even if slight, can lead to perceptible tone variation in the scan line direction of the printed image, commonly referred to as banding artifacts. FIG. 5 shows light and dark streaks within an image 18 which represent banding artifacts in that image as printed. Furthermore, the perceptibility of such banding increases in multiple-color printing systems due to color mixing and the ability of the human eye to accurately detect certain nonlinearities in color gradation. Another common imaging quality issue is the stair-step pattern produced when attempting to print a curve, commonly known as “jaggies”.
One approach to increasing image quality from an electrophotographic marking system is to simply increase the number of laser sources forming the integrated array to increase the scan resolution. However, while an integrated array with 32 individual sources is currently a reasonably standard, readily available device, integrated laser arrays with more than 32 sources are not. Thus, any system incorporating an integrated array of more than 32 sources must account for the significantly increased cost of a specially designed and built laser array. In addition, as one adds sources to an array either the source spacing shrinks, making array fabrication more difficult and costly, or the optical system aperture decreases relative to the output beam divergence of the laser due to a smaller required optical magnification, thus requiring an increase in per-laser power. Higher powered lasers run hotter, have a shorter lifespan, and are again non-standard. Furthermore, for each additional source in an integrated array there is a corresponding increase in the risk of a device failing and rendering the entire integrated array device non-useable.
If one were to simply abut two or more integrated laser arrays and direct the beams they produce to a single spot on the scanning subsystem for scanning, as disclosed for example in the aforementioned U.S. Pat. No. 7,236,280, the beams from each array will travel in different optical paths. Inherent operating variations such as thermal changes result in different displacements for the different optical paths, resulting in visible printing artifacts in the final printed image.
Thus, there are a number of compelling reasons that simply increasing the number of sources in an integrated array as well as simply abutting two integrated arrays together and directing their output to the scanning subsystem are not practical responses to the demand for increased resolution.